Chemical is the new thermal: Thermodynamic efficiency in dissipative chemistry

Out of equilibrium experiments with chemical systems are playing nowadays a role similar to thermal machines during the industrial revolution, at least in the way they challenge physicists. Time is ripe to set up a proper thermodynamic framework to characterize performance in such chemical engines.

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The
seeds our work ripened from were spread in the late 2016. At that
time the editorial board of Nature Nanotechnology raised
the
need “to connect supramolecular chemistry to the out-of-equilibrium
thermodynamic concepts being developed by theoretical physicists”,
since
“a firm grasp of the physical chemistry of out-of-equilibrium
systems” was
lacking [Nature
Nanotech.
11, 909].

Meanwhile
Massimiliano
and Riccardo (my two co-authors)
had
already embarked
on
establishing
a
rigorous
theoretical
framework
describing
thermodynamics of open chemical reaction networks,
beyond
the
typical close-to-equilibrium assumption and in terms of macroscopic
concentrations [Phys.
Rev X
6, 041064].
Actually,
that paper was
one of the reasons
why I decided to apply for a PhD in Luxembourg University!

Despite
the
fertile
ground,
germination
only
started
in the spring of 2018 during a conversation
with Dr.
Giulio Ragazzon. He
was working together with Prof. Leonard Prins on chemical fuel-driven
self-assembly
in supramolecular chemistry
[Nature
Nanotech.
13,
882],
where
high-energy molecules can
accumulate because
energy
get
consumed
(see
figure).
From
the
point of view adopted
in our group,
they
were dealing with open chemical reaction networks where work is
performed to induce a time-dependent change in the species abundances
that could never be reached at equilibrium. In
other words, the first stroke of a chemical
engine!

Suitably engineered monomers (middle) can spontaneously assemble into ordered low-energy structures (left). This relaxation towards equilibrium can be relatively well controlled by chemists. On the other hand, exploiting an energy source to induce the assembly of an energy-demanding structure (right) is a frontier challenge. How to quantify the efficiency of such nonequilibrium operation? (I thank Giulio for the picture)

By
looking
at open chemical reaction networks
as chemical engines it
became clear that we could use our theory
to analyze
and optimize
their performance. Following
on
the heels
of early thermodynamics for
steam engines, we
developed
general
and
quantitative notions
of efficiency and
work
valid
for any dissipative chemical process.
We
used
them to analyze
two paradigmatic
models
inspired by supramolecular chemistry: energy storage – the time
dependent accumulation
of high energy species far from equilibrium – and driven synthesis
– the continuous
endergonic
assembly
and
extraction
of monomers
which
scarcely
bind
at equilibrium. For
this latter,
we found
working conditions in which the system simultaneously
maximizes efficiency and power, a very rare condition in the
macroscopic world which may reveal
a keystone in nature.

Since
many of the functionalities of organisms
are ultimately powered by chemical reactions,
we hope that the
fruits of our work
will be helpful
for system
biology. Cells
can indeed be thought of as highly
evolved chemical engines
which burn some fuel (usually ATP) to perform operations
such as
maintaining a nonequilibrium state or assembling
molecules affording moving (e.g.
microtubules) and
other tasks.

The
way towards a proper
understanding of energy
processing in
biology and the
development of new generations
of chemically designed engines lies
at the interface between physics, chemistry and biology. For
this reason, we
wrote the manuscript using a language
accessible to people from
different backgrounds: we
believe that crossbreeding activities
among various
communities are
important to make the field of dissipative
chemistry a more
and more blooming one.

The
work was funded by the Luxembourg National Research Fund (FNR) and
the European Research Council (ERC) project NanoThermo. I am also
extremely thankful to Michela Bernini and Mitch Woodensteel for their
artistic interpretation of our research in the poster
image!

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